A problem of rapid and accurate positioning arises when it is desired to place a photographic plate in a working position on a system of mobile tables with cross-feed motions. These tables are utilized, for example, in photographic reproduction apparatus such as a photo-repeater, i.e. a machine for the multiple reproduction of integrated-circuit masks. In an apparatus of this kind, the tables, in moving, are to remain parallel to a fixed (usually horizontal) reference plane. The reference trihedral system is here represented by the optical axis of the apparatus and by a system of rectangular coordinates located in the plane of one face of one of the mobile tables (perpendicular to the optical axis). The desired precision, in the case of integrated circuits, is on the order of one micron.
The known solutions to the problem of precise guidance can be split into two main categories:
1. Direct positioning of the workpiece upon the top face of a mobile table. The photographic plate is then immobilized by simple stops. Under these circumstances, it is the bottom face of the plate, resting on the mobile tables, which constitutes a reference plane. However, the parallelism of the faces is only approximate in the plates currently used: the photosensitive film on top will therefore not be parallel to the reference plane within one micron, in the manner which the technology of integrated circuits requires. It will therefore be necessary in this case to utilize specially manufactured plates, which do have this degree of accuracy, and this constitutes a drawback.
2. Positioning through the medium of a plate-holder frame. The photographic plate is then immobilized by supporting its top face, that is to say the photosensitive film, against the edges of that frame bounding the plate. The plate is permanently subjected to an upward supporting force, for example by means of a leaf spring applied against its center. In this solution, even if the frame is designed with sufficient accuracy to ensure parallelism of the edges with the reference plane, there will be a slight distortion of the plate under the effect of the supporting force, so that a positional error, substantially in excess of one micron at the center thereof, will generally occur.
Finally, a common drawback to both the aforementioned solutions arises in the quite general case where the plate, or the plate-holding frame, rests against simple stops attached to a mobile table so as to shift inertially if the moving table executes sudden displacements. Naturally, these movements could be inhibited by employing a system of elastic stops but, because of the stress produced in the section of the plate, distortion of the latter is inevitable.